In a known oscillator of this type, the resonant optical cavity is formed by the first and second reflectors. The output laser beam passes through the opening in the second reflector and leaves the enclosure by passing through a transparent window made in the wall of the wall of the second chamber. However, a considerable portion of an axial cylindrical zone in the first chamber of cross section corresponding to the opening in the second reflector is not occupied by the oscillating light rays, and this results in an appreciable loss of output power.
To remedy this drawback, a proposal has been made to use a multipass cavity comprising a plurality of reflectors, (eg. 14) divided into two groups. Oscillating rays are set up between these reflectors forming a zig-zag beam which passes through nearly all of the inside volume of the first chamber. However, this arrangement leads to high light losses by reflection on the numerous mirrors in the cavity, resulting in an increase in output power which is much smaller than the expected increase.
Preferred embodiments of the present invention provide an improved laser oscillator of the above-defined known type, in which the output power is increased very appreciably.